The growth of materials directly on two-dimensional inert materials (2DIM), for example graphene, is challenging due to the lack of out-of-plane or dangling bonds on the surface to promote bonding with the foreign atoms.
As a result, we have employed various functionalization methods to modify the surface to promote bond formation during low temperature atomic layer deposition of high-k dielectrics on graphene and have resulted in pristine dielectric/graphene interfaces without altering the advantageous graphene electronic properties. The preferred method of functionalization is the chemisorption of an atom forming a semi-ionic bond to the 2DIM surface that preserves the structural and electrical integrity of the 2DIM, while providing sufficient nucleation sites for subsequent layer deposition. Fluorination, using xenon difluoride (XeF2) gas, is one of the methods shown to functionalize the graphene surface with little degradation to the graphene lattice. In fact, an increase in mobility after the deposition of a thin oxide on fluorinated graphene with 6-7% of C—F bond has been shown. Similar approaches/arguments can be made for the whole family of 2DIMs.
High temperature growth of III-nitrides under conventional molecular beam epitaxy (MBE) or MOCVD conditions (500-1300° C.) directly on functionalized graphene would result in complete desorption of the semi-ionically bonded fluorine (or alternative) adatom and could hinder the advantage of fluorination, thus a low temperature albeit high quality growth of a III-N nucleation layer is important.
In order to preserve the essential fluorine functional groups, atomic layer epitaxy (ALE) is the preferred growth method as it enables growth of thin, uniform crystalline layers at low temperatures. The inherent kinetics of ALE, allow the growth of crystalline materials at greatly reduced temperatures relative to standard epitaxial techniques such as MOCVD.
In this disclosure we report on the first ever epitaxial growth of III-N semiconductor layers on graphene—a key enabler to a range of wide band gap-2D material heterojunction devices.